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Non-return valves

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Non-return valves are essential devices for the satisfactory operation of high pressure piston pumps. Essentially, the non-return valve is a one-way valve that allows a liquid to flow through it in one direction but completely prevents its movement in the opposite direction. A piston pump is fitted with two non-return valves one at the cylinder inlet that allows liquid to flow into the cylinder but does not allow it to flow out. The other non-return valve is fitted to the cylinder outlet that allows the liquid to flow out of the cylinder, but does not allow it to flow back into the cylinder. Thus, on the forward stroke of the piston, liquid is forced out of the cylinder to the column but can not flow back through the refill valve. On the return stroke, liquid is prevented from flowing back to the cylinder from the column but liquid is allowed to flow in through the refill port into the cylinder. The non-return valve consists of a ball and seat, the ball usually being made from synthetic sapphire, the seat being made of stainless steel or, also from sapphire. Liquid flowing from the seat-side of the valve lifts the ball from the seat and allows a flow of liquid through the valve. Liquid attempting to flow from the ball-side of the seat forces the ball back against the seat which completely blocks the flow of liquid through the valve. Modern non-return valves are very efficient allowing almost zero flow of liquid through the ball and seat seal.
The piston is driven by a stainless steel cam which forces the piston into the cylinder expressing the solvent through the exit non-return valve. After reaching the maximum movement, the piston follows the cam and returns as a result of the pressure exerted by the return spring. During this movement the cylinder is loaded with more solvent through the inlet non-return valve. The shape of the cam is cut to provide a linear movement of the piston during expression of the solvent but a sudden return movement on the refill stroke. In this way the pulse effect that results from the refill action is reduced. The pulses, however, are not completely eliminated and the detector noise resulting from these pulses is probably the most serious disadvantage of the single piston pump.


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